This invention relates to the manufacture of superconductors in general and more particularly to an improved method of manufacturing a superconductor having a superconductive intermetallic compound made up of at least two elements.
Intermetallic superconducting compounds consisting of two elements of the type A.sub.3 B, for example, Nb.sub.3 Sn or V.sub.3 Ga, and having an A-15 crystal structure are known to have very good superconductive properties and are distinguished in particular by a high critical magnetic field, a high transition temperature and a high critical current density. As a result, compounds of this nature are particularly well suited for superconducting coils used for generating strong magnetic fields such as those used for example, for research purposes. In addition, such coils find application in the superconducting magnets used in the suspended guidance of magnetic suspension railroads or in the windings of electric machines. More recently, ternary compounds such as, niobium-aluminum-germanium (Nb.sub.3 Al.sub.0.8 Ge.sub.0.2) have become of particular interest. However, since these compounds are very brittle, considerable difficulty is encountered in their manufacture into a form suitable for use in magnet coils or the like.
One previous attempt at making suitable conductors comprise making wires and ribbons with surface layers of Nb.sub.3 Sn by applying tin to a niobium wire or niobium ribbon electrolytically, by evaporation or some similar means and then diffusing it into the niobium through subsequent annealing at temperatures of between approximately 950.degree. and 1000.degree. C where the tin reacts with the niobium to form an Nb.sub.3 Sn layer. With these processes, the wires or ribbons were then coated with a copper layer for electrical stabilization. Generally, this was done electrolytically. In another method which attempts to overcome some of the difficulties with with this method, a niobium layer is applied to a copper core to produce a stabilized ribbon or wire shaped superconductor with a niobium-tin layer on the surface. The niobium tin layer is formed by diffusing tin into the niobium in a well known manner. Such is disclosed in German Pat. No. 1,282,117.
These prior art methods, however, have various disadvantages. Primarily, they require relatively high diffusion temperatures. In addition, the manufacture of what is referred to as multi-core conductors with a multiplicity of thin, superconducting filaments embedded in a normally conductive matrix, caused considerable difficulties. This was due to the brittleness of the intermetallic compounds. Several further methods were developed, making possible the manufacture of superconductors with, in particular, two-component intermetallic compounds in the form of long wires or ribbons at low diffusion temperatures. These methods which are particularly applicable to the manufacture of multi-core conductors using wires of Nb.sub.3 Sn and V.sub.3 Ga, embedded in a normal-conducting matrix, are carried out by surrounding a ductile element of the compound to be produced in wire form, such as a niobium or vanadium wire in a sheath of an alloy containing a ductile carrier metal and other elements of the compound, e.g., a copper-tin alloy or a copper-gallium alloy. In particular, multiplicity of such wires can be embedded in a matrix of the alloy. The structure so obtained is then subjected to a cross section reducing process. This results, on one hand, in a long wire such as that required by coils, and on the other hand, the reduction of the diameter of the wire which is, for example, niobium or vanadium to a low size in the order of about 30 to 50 .mu.m. This is desirable in view of the superconductive properties of the conductor. In addition, the cross section reducing process obtains the best possible metallurgical bond between the wire and the surrounding jacket material of the alloy, without the occurance of reactions that lead to an embrittlement of the conductor. After the cross section-reducing process, the conductor, which consists of one or more wires and the surrounding matrix materal, is subjected to heat treatment in such a manner that the desired compound is formed through the reaction of the wire material such as niobium or vanadium with the additional element of the compound contained in the surrounding matrix. This additional material would be for example, tin or gallium. During this process, the element contained in the matrix diffuses into the wire material and reacts with the latter forming a layer consisting of the desired compound. Processes of this nature are disclosed in German Offenlegungsschrift 2,044,660, German Offenlegungsschrift 2,052,323, and German Offenlegungsschrift 2,105,828.
This process, however, suffers from a serious drawback. During diffusion, the entire element of the compound contained in the alloy sheath never diffuses into the wire or wires of the other element of the compound. Instead of being consumed in forming the compound, considerable quantities of this element remain in the matrix due to the diffusion conditions. This results in a relatively high electrical residual resistance in the portion of the matrix material containing some of the compound. For example, the residual resistance of copper increases quite steeply with an increasing gallium content. As a result, the sheath is not well suited as a stabilizing material for the superconductor. Electrical stabilization of the superconductor is, however, required as a rule, in order to prevent a sudden transition of the superconductor from the superconducting to the electrically normally conducting state. As is well known, the superconductor, in order to be stabilized, must be brought into intimate contact with a metal which is electrically and thermally highly conductive and is electrically normally conducting at the operating temperature of the superconductor, e.g., at 4.2 K. In addition, the stabilizing material must be able to rapidly remove the heat which is produced by the temporary local occurrence of normal conduction in the superconductor. This heat must be removed therefrom and transferred into the coolant such as liquid helium which surrounds the superconductor. In addition, the stabilizing material must be capable of taking over, at least for a short time, the current which normally flows through the superconductor, should a local occurrence of normal conduction come about. Copper, aluminum or silver, preferably in a highly purified form, as well suited as stabilizing materials.
Thus, it can be seen that all of the prior art processes for making intermetallic superconducting wires have drawbacks. The need for an improved process which avoids the above noted problems is therefor evident.